1. Field of the Invention
The present invention relates to a reactive sputtering system and more particularly to a reactive sputtering system having a plurality of gas distribution systems.
2. Description of the Prior Art
The reactive sputtering process is a thin film forming process wherein a target material is sputtered while supplying a reactive gas to a processing chamber, and a reaction product of the sputtered target material with the reactive gas is then deposited on a surface of a substrate to form a thin film. FIGS. 1A and 1B are partially schematic views of a reactive sputtering system constructed in accordance with the prior art, in which FIG. 1A shows a side view of the reactive sputtering system and FIG. 1B shows a plan view of the same wherein a substrate is omitted.
In the prior art sputtering system, a mixture of the reactive gas with argon gas is introduced into the processing chamber through nozzles (not shown in FIGS. 1A and 1B) of a gas distribution pipe 3 in a constant ratio. When plasma is generated between a surface 1 of the substrate on which a thin film is to be formed and a target 2 mounted on a backing sheet 7, ions of the argon gas and the reactive gas collide against the surface of the target 2. As a result, the material of the target 2 is sputtered to deposit the thin film on the surface 1 of the substrate.
More exactly, the material of the target 2 is sputtered when the ions of the reactive gas and the argon gas collide against the surface of the target 2 or after collisions of the ions against the surface of the target 2. Thereafter, the sputtered target material travels through the plasma area in which the material is allowed to react with the ions of the reactive gas. A reaction product thus obtained is deposited on the surface 1 of the substrate.
In the above-mentioned prior art sputtering system, the concentrations of argon and of the reactive gas are maintained uniform outside of the outer periphery of the target. Within the plasma area, however, the argon gas and the reactive gas are ionized. The reactive gas travels, in passing through the plasma area while the reactive gas is allowed to react with the sputtered target material. Thus, the concentration of the reactive gas varies in the radial direction of the target 2. This leads to a difference in reaction rate.
The difference in reaction rate causes the composition of the film deposited on the surface of the substrate to vary so as to deteriorate the film properties along the substrate surface. Eventually, this will cause scattering in the properties of semiconductor devices in which such films are used and reduce the yield of the semiconductor devices, with a reduction in their reliability.